Steam generating systems inherently produce condensate in high pressure lines that must be removed to prevent erosion, corrosion, and other damage to the system. The removal of this condensate from high pressure steam systems by known methods inevitably results in a heat loss to the system and efforts have been made to reduce the magnitude of this loss to increase the efficiency of the system and conserve energy.
For many years steam traps have been employed for this purpose, but because they malfunction frequently and do not exert any significant back pressure on the system, they permit an excessive amount of live high pressure steam to escape to drain where it condenses without producing any work resulting in a significant system loss. Moreover the filters commonly associated with these traps provide a frequent source for the entrapment of foreign material and require periodic cleaning and replacement.
The field of the invention relates to the art of restricting the flow of water and other liquids by the means of providing a fixed restriction in a conduit. As is well known in the art, choking of liquid flow through a restriction can be achieved by accelerating the liquid to that velocity at which it flashes into a two phase mixture of liquid and vapor. The mixture can thus achieve a velocity equal to the speed of sound in the mixture. When sonic velocity has been obtained, the flow rate no longer depends on the downstream pressure, but only on the upstream pressure. This phenomena is known as sonic choke. Such a device has useful application in many areas such as where condensate is to be removed from steam heating systems.
The principles of gaseous (compressible) choked flow have been understood for some time, and more recently the principles of two phase choked flow have also been formulated.
In order to achieve choked flow in a liquid, the restriction must be relatively small. Although there is no theoretical limit to how small the restriction can be made to obtain a certain flow, it is well known that if a restriction is too small, it will become plugged by impurities in the fluid. In addition it is well known that a flashing liquid is very erosive to such restrictions.
There have been two general approaches that have been taken to overcome the risk of plugging, or to lessen the effects of erosive forces:
(a) Instead of one very small restriction, larger, multiple restrictions are provided to break the pressure in more than one step. This is the approach taken by Self in U.S. Pat. No. 3,954,124, by Kuehn in U.S. Pat. No. 3,983,903, and by Voorheis in U.S. Pat. No. 3,409,382.
(b) A single, small restriction is provided but with a protective screen in front of the restriction. This is the approach taken by Wonderland in U.S. Pat. No. 3,887,895, Lee in U.S. Pat. No. 3,109,459, Mannion in U.S. Pat. No. 3,668,822, and Kasten in U.S. Pat. No. 2,635,641.
The disadvantage of design (a) above is that it is a relatively complex, costly device to manufacture and install. It is also difficult to disassemble for inspection and maintenance. It is also difficult to resize this device in the field should the need arise.
The disadvantages of design (b) above is that the screen itself is somewhat fragile and is subject to damage by erosion or corrosion. Furthermore, the screen openings are so small that they are prone to plugging. Another disadvantage is that this type of device is normally attached by gasketing or threading and runs the risk of leaking to the atmosphere. The devices requiring gaskets (such as U.S. Pat. No. 3,887,895) also require an inventory of the gaskets which must be replaced periodically and/or at each inspection. Another disadvantage of this type of device is that a single, thin orifice plate (U.S. Pat. No. 3,877,895) is vulnerable to the erosive forces.